<!DOCTYPE html>
<html>
    <head>
        <script src="js/vue.js"></script>
    </head>
    <body>
        <div id="app">
            <input type="text" v-model="cityName">
        </div>
        <script>
            var vm = new Vue({
                el:'#app',
                data : {
                    cityName:'shanghai'
                },
                watch : {
                    cityName(newName,oldName){
                        console.log(newName,oldName);
                    }
                }
            });
        </script>
    </body>
</html>